Length and weight
Composition of mackerel
Chilled and frozen storage
Mackerel and poisoning
This note gives information on the shelf life of chilled and frozen mackerel and describes the manufacture of smoked, canned and salted products from mackerel.
The processing advice is preceded by a brief description of the fish, its life history, and the fishery in the UK, with information on the quantity and value of the catch.
The scientific name of the mackerel caught in the north Atlantic is Scomber scombrus. The scientific name of the chub mackerel, a related species found in warmer waters in many parts of the world, is Scomber japonicus. Other species of Scomber, and species of Scomberomorus, Auxis and Rastrelliger are also described as mackerel in many parts of the world.
The name mackerel is used in the UK principally for the species Scomber scombrus, but the name can also be used in the UK for any species of Scomber offered for retail sale.
Scomber scombrus is sometimes called Atlantic, northern or Boston mackerel to distinguish it from Scomber japonicus, which is variously called chub, Pacific, or Spanish mackerel.
A small mackerel is sometimes called a cock mackerel, a joey or a little boy in England, or a pinner in Scotland.
The Atlantic mackerel has a rounded, elongated body with two widely spaced back fins, the first with 11-13 spiny rays, the second with soft rays; the chub mackerel has 9-10 spines in the first back fin. There are 4-6, usually 5, finlets between the second back fin and the rail, and between the anal fin and the tail, There is a small keel on each side of the tail stub. The tail is forked.
The back of the mackerel is a brilliant greeny blue, and the head is a steely blue-black with a small yellow patch behind the eye. There are 23-33 dark wavy bands across the back of the fish down to the midline. The sides have a silvery or coppery sheen, the belly is silvery white and there is a broken black line just below the lateral line.
The scales are small and the skin feels velvety. There is a swimbladder in the chub mackerel but not in the Atlantic mackerel.
Mackerel are found in summer throughout the waters of the European continental shelf, continually on the move in search of food; they feed on small fish such as sprats and sand eels, as well as on krill and plankton. At the start of the winter the mackerel migrate to overwintering areas; these are not well defined, but three are known around the UK, one in the northern North Sea along the Norwegian Deeps, another along the edge of the continental shelf in the western Celtic Sea and a third close to the south coast of Cornwall. The first two are typical overwintering areas and are similar to those off the east coast of North America; the mackerel are mixed with other species, for example scad in the Celtic Sea and Norway pout in the North Sea. Off Cornwall the pattern is different; large shoals of mackerel with few other fish among them move close inshore into relatively shallow water, although separate shoals of pilchard and scad are found in the area. The Cornish stock of winter mackerel was unknown before the mid 1960s, presumably because it was not there, and it is assumed to be an unstable offshoot of the stock in the Celtic Sea. Should the unknown factors that attracted the shoals suddenly change, there is always the possibility that they may just as suddenly withdraw from the area; nowhere else in the north Atlantic are adult mackerel found in a comparable position.
Mackerel do not feed while overwintering, but they are not totally inactive. They form large dense shoals near the sea bed during daylight, but rise and disperse during darkness. Occasionally they disperse for no obvious reason and reform hours or even days later many miles from their previous position.
Mackerel begin to move away from the overwintering areas in the spring to spawn and to start their feeding migrations. Spawning begins along the edge of the continental shelf in southern Biscay during February. By late March spawning occurs throughout the Bay of Biscay, the Celtic Sea and to the west of Ireland, and continues until June, but at all times the greatest concentrations of eggs are found along the edge of the shelf. The older, larger fish start spawning first and, when spent, start swimming northwards along the edge of the shelf towards Shetland. Smaller, younger fish start spawning later and do not migrate so far. The youngest fish remain in the Celtic Sea throughout the summer. The spawning season in the North Sea is later, from May to August, and is most productive north of the Dogger Bank. Those that spawn south of the Dogger Bank are part of the western overwintering stock that spawns and feeds in the southern North Sea. Once spawning is completed the North Sea mackerel mix with the older fish from the western stock around Shetland until the winter migration begins.
Each female mackerel spawns on average about 300 000 eggs which, when fertilized, are about 1 mm in diameter. They each contain a large oil globule which helps them to float in the upper water layers until they hatch. The larvae, 2-3 mm long, hatch after 4½ days at 15°C or 8½ days at 10°C, and by the end of the first year the young mackerel are 15-20 cm long. Growth continues to be rapid until they reach maturity at 3-4 years, when they are 28-34 cm long. Growth rate is then much reduced, and mackerel 35-40 cm long may be anywhere between 5 and 20 years old. While a large fish is usually an old fish, one mackerel 57 cm long was found to be only 6 years old.
An adult mackerel 30-35 cm long weighs 300-500 g before gutting. A joey mackerel, 20-25 cm long and probably in its second year, weighs 100-200 g. The graph shows the relationship between length and weight of gutted and ungutted mackerel. Mackerel weighing more than 1 kg are exceptional.
Fig. 2 length and weight of mackerel
The mackerel is found in the north east Atlantic from Norway to Morocco and the Canaries, and in the Mediterranean and Black Seas. In the north west Atlantic it occurs from Labrador to North Carolina.
Large dense shoals may be found near the surface all round the UK in summer and autumn, but their occurrence is sporadic in the southern North Sea. The English summer mackerel fishery was traditionally from April to September; this has now been extended from February to October so that, coupled with the huge winter fishery now established in the south west from October to March, mackerel supplies are available throughout the year. The Scottish mackerel fishery from June to August could also possibly be extended.
Mackerel fishing methods have changed dramatically over the past 50 years; before 1939 mackerel were caught mainly in the spring by drift net, an interim occupation for drifters between the winter and summer herring fisheries. After 1945 mackerel landings were small for some years, and the catch was taken mainly in the south west during the summer by feathering, that is small boats using hand lines with coloured lures made from feathers or plastics. Once the mackerel shoals were found to remain in Cornish waters throughout the winter, the hand line catch increased considerably in the late 1960s, and, as the market for mackerel improved, trawlers began to engage in mackerel fishing, using both midwater and bottom trawls. Purse seining has become the most important method of capturing mackerel in quantity off the west of Scotland in late summer.
The mackerel is a fatty fish, and the fat and water content vary with season. The fat content of mackerel caught off south west England is lowest in May after spawning, and reaches its peak between October and December after the fish have fed during the summer and autumn. A typical range of fat content throughout the year is 6-23 per cent. As fat content increases, water content decreases; the typical range of water content is 56-74 per cent. The protein content is 18-20 per cent. The fat content of mackerel caught off the west of Scotland during August and September is similar to that of Cornish mackerel during December and January.
Mackerel are fatty fish and are not normally gutted at sea; they therefore spoil quickly unless they are chilled immediately after catching and kept chilled. Mackerel with a fat content of about 10 per cent will develop off odours after 1-2 days at 10°C, will be soft and spoiling rapidly after 3 days, and will be putrid after 5-6 days; fish with a higher fat content spoil even faster.
Mackerel of medium fat content stowed in ice or in refrigerated sea water immediately after capture will keep in good condition for 4-5 days.
Stale or noticeably spoiled mackerel should not be used for processing. The signs of stale fish are as follows:
Eyes are sunken, cloudy and discoloured red or brown.Mackerel for freezing should be chilled immediately after capture and frozen within 24 hours; mackerel with a high fat content should preferably be frozen within 12 hours. Frozen mackerel, properly glazed and kept in cold storage at -30°C, will keep in good condition for at least 6 months. Cold storage life can be extended up to 1 year when mackerel are packed in polyethylene bags that are topped up with water and frozen in vertical plate freezers. The block is doubly protected against deterioration in store by the encasing ice and by the packaging. Gutting before freezing is not necessary, and may be a disadvantage.
The skin has lost bloom, and its colours have lost intensity and brilliance, giving a washed out appearance.
Gills are dark red or brown, and a dark blood red mucus oozes from the gill covers.
The odour of the gills and body is sour, sweaty or strongly oily; sometimes a smell of ammonia is also present.
Fig. 3 seasonal variation in composition of Cornish mackerel
Mackerel can be hot smoked to give a ready-to-eat delicatessen product, and the flesh of hot smoked mackerel can be used as a basis for a pate. The mackerel can also be split or filleted and then kippered in cold smoke in a manner similar to herring.
Hot smoked mackerel
For a product of good eating quality, mackerel with a fat content of at least 10 per cent should be used. The smoked product can be whole gutted fish, with or without the head on, or fillets. For smoked whole fish, the mackerel are gutted, or headed and gutted, as required, the gut cavity cleaned and the black bellywall lining removed. For hot smoked fillets, single fillets with skin left on are cut from the whole fish.
Brining requires some care. In order to minimize the risk of food poisoning organisms growing in the finished product it is recommended that the salt concentration in the water phase of the product should be at least 3 per cent; this concentration expressed in terms of water plus the solid components of the flesh is less than 3 per cent. To measure the salt concentration in the water phase it is also necessary to measure the water content of the product. A salt concentration of 3 per cent does not render hot smoked mackerel unacceptably salty to the consumer.
The rate of uptake of salt during brining depends principally on the size of the fish; large fish take longer to brine than small ones. Thus to achieve a reproducible uptake of salt all the fish in a batch should be of about the same weight.
A gutted mackerel of average size and fat content has to remain in the usual 80° brine for 5 hours before the salt concentration reaches the recommended 3 per cent, making it difficult to complete the whole process of gutting, brining and smoking in a normal working day. As an alternative the use of a weaker brine for a much longer period allows the fish to be left in brine overnight. As a guide, the following brining conditions give a satisfactory product.
Weight of headed, gutted fish
Brined whole fish can be tied by the tails in pairs and hung over tenter rods for smoking, or hung individually by the tails in keyhole slots cut in metal frames designed for the purpose as shown in the drawing. Mackerel with heads on can be speated through the eyes. Fillets are laid on trays made of nylon coated or stainless steel wire mesh. Full tenters, speats or trays are loaded on trolleys which when fully loaded can be left to drain for 1-2 hours or wheeled directly into the kiln without draining. In kilns with more than one trolley, the first loaded trolley should be placed at the air outlet end, and the last loaded trolley at the air inlet end.
The smoking procedure for whole mackerel in a mechanical kiln is as follows. With the kiln thermostat first set at 30°C, the air inlet half to threequarters open, and the main fan and chimney fan on, the smoke producer is made to produce maximum smoke, the recirculation damper being set to balance the smoke pressure in the kiln. This predrying and smoking period takes ¾-1 hour, during which time surplus moisture dries off the surface of the fish, and the skins begin to set. The thermostats are then reset at 50°C, the air inlet reduced to a quarter open, and the smoke pressure balanced by resetting the recirculation damper. Temperature and humidity in the kiln begin to rise, and the fish start to cook; high humidity prevents excessive weight loss. Once the temperature reaches 50°C it should be kept at that for ½ hour. The skin of the fish now feels firm and colour begins to develop. For the final cooking stage the order of the trolleys is reversed and each trolley is reversed; in a single trolley kiln the trolley is reversed. The thermostats are reset at 80°C, the air inlet closed and the recirculation damper opened only just enough to relieve any back pressure on the smoke producer. By the time the kiln temperature reaches 80°C, small mackerel will need a further 40-45 minutes to complete the process; larger fish need about 75 minutes. As a rough guide, the whole smoking process should take about 3 hours. The fish attain a dark golden brown colour, and the skin is dry with a silky sheen. The flesh should be completely cooked; the thickest part of the flesh at the shoulders of a whole fish should be opaque, with no jellylike flakes.
Gutted mackerel may be split down the back in the same way as herring are prepared for kippering. A split mackerel of average size and fat content requires 7 minutes in 80° brine. The fish may also be cut as single or block fillets; brining time is 3 minutes in 80° brine. The cold smoking process for kippered fish is described in detail in Advisory Note 48; mackerel fillets require about 2½ hours, split fish about 4 hours in a mechanical kiln at 30°C.
A delicious pate can be made from the flesh of hot smoked mackerel, either on a commercial scale or in small quantities for domestic use. The flesh is taken off the whole smoked fish, and all bones are carefully removed. For commercial production this can be done satisfactorily in a mechanical separator.
The flesh is mixed with softened or melted butter in a high speed blender, using 7 parts by weight of fish to 3 parts of butter. When the mixture is of smooth consistency, flavouring or spices may be added if required; for example lemon juice and pepper can be used. Manufacturers can readily devise their own variations on the basic recipe. Some white fish flesh is sometimes added to compensate for variations in the fat content of the mackerel flesh in the mix.
The product is not sterile, and should not be kept more than 1 day at ambient temperature, or more than 5 days at 0-2°C. The pate can be frozen and cold stored in suitable containers, or it may be canned after adding a stabilizer.
Mackerel may be canned as steaks, as fillets in a variety of sauces, or as a cold smoked product.
Canned mackerel steaks are prepared in the following way. The whole fish are headed and gutted, and the belly cavity cleaned out. The black skin is removed from the belly wall, and traces of blood along the backbone are brushed away. The fish are then cut into steaks of a length to suit the pack; for example steaks 6-7 cm long are suitable for a 220 g cylindrical can. The pieces are packed vertically in the can to give a product that resembles a middle cut steak of a larger fish like tuna or salmon. After adding 3 g of salt, the can is closed and heat processed for 90 minutes at 115°C. Vegetable oil, mackerel oil or tomato sauce can be added to the can before closing, or the pack can be left dry to suit particular markets. The finished cans should be stored for about 1 month before labelling and distribution; there is no further significant change in flavour or texture during storage after that time.
Small mackerel can be steaked for canning, but the higher ratio of bone to flesh sometimes makes the texture of the finished product slightly gritty.
Mackerel fillets or pieces for canning in a sauce are first soaked in saturated brine for 6 minutes, then packed into 200 g oval or ring-pull cans and steamed for 10 minutes. Any liquor in the can is then poured off, the can is filled with hot sauce, closed and heat processed for about 60 minutes at 115°C, the time depending on the ratio of fish to sauce. A variety of sauces can be used; port, cider and sweet-and-sour sauces have been found acceptable during trials.
Mackerel fillets are smoked for canning in the following manner. Block fillets are soaked for 2-3 minutes in 75° brine containing 0·5 per cent vinegar, drained for 1-2 hours and then smoked in a mechanical kiln for ½ hour at 30°C and for a further 1 hour at 50°C, using maximum smoke. The fillets will lose about 11 per cent of their weight during smoking. The hour at 50°C is necessary to fix the colour of the fillets which would otherwise darken to an unacceptable degree during heat processing in the can. The smoked fillets are packed in 200 g oval or ring-pull cans and covered with a vegetable oil; maize oil has been found most suitable. The cans are clinched, exhausted in a steam chest for 10 minutes, closed and then heat processed for 60 minutes at 115°C.
Quantities of mackerel were at one time pickle cured in barrels for export, and small amounts are still occasionally packed in this way. A typical process is briefly described.
Fresh mackerel are split along the back, and opened with a jerk to break the ribs. Guts, gills and any protruding bones are removed, and blood is washed away from the backbone. The fish are soaked in fresh water for up to 2 hours, rinsed, drained and then dipped in dry salt. The fish are packed in barrels in circular fashion with tails to the centre, and salt is sprinkled on each layer. The first two or three layers are packed skin downwards, and succeeding layers skin upwards. About 1 kg salt to 3 kg fat mackerel or 4 kg lean mackerel is required throughout the pack. When the barrel is full it is closed, laid on its side, topped up with brine through the bung and left for 10-12 days, further brine being added if required to keep the barrel full. The fish are usually graded and repacked for shipment; the fish are repacked in the barrel in the same way, but with less salt, about 1 kg salt to 6 kg fish. The barrels are topped up with 95° brine, the bungs are closed and the barrels set on end for labelling and dispatch. Fillets of mackerel can be pickle cured in barrels in the same manner as split mackerel.
There is a belief that people can become ill as a result of eating mackerel and that this is in some way associated with mackerel being a dirty feeder. The truth is that the diet of the mackerel, which is little different from that of many other commercial species, has no effect on the wholesomeness of the fish. There is no evidence that normal mackerel or mackerel products cause illness.
A certain kind of relatively mild food poisoning is known to
occur, particularly in warm countries, when spoiled mackerel and other scombroid
fish like tuna and saury are eaten. If there is any suspicion, therefore, that
mackerel are noticeably spoiled they should not be eaten. The signs of spoilage
are described earlier in this note.